Conan O'Brien made a surprise appearance on Late Night with Jimmy Fallon last night ... and it was the first time the comedian has been back to NBC since his major fall-out with the peacock network back in January 2010.

Jimmy now works on the same New York City sound stage that Conan called home for sixteen years. "You know, I was out in New York, I decided to drop by and check out my old studio," Conan told Jimmy after the audience's cheers died down.

"You were here for sixteen years. And then what happened?" Jimmy quipped.

After a hearty laugh, Conan -- who is now based in Los Angeles -- replied:

"Don't you worry about that. You're a young guy ... it worked out fine. Anyhoo, I came by for a reason, which is I left something here, something important to me. And I haven't had it in a while and I thought since I'm here ... I can get it back. Is that cool with you?"

What was it? Conan's famous prop pup, Triumph the Insult Comic Dog. See how Triumph fared in the video above.

Reunited and it feels so good!

The famous red head hosted Late Night with Conan O'Brien from 1993 to 2009. He made a deal in 2004 to take over for Jay Leno as the new host of The Tonight Show in 2009.

Jimmy replaced Conan as the Late Night host in March 2009, but O'Brien's Late Night gig lasted less than a year, when Jay Leno came back to host his original show in the spring of 2010. The entire fiasco led to a major dispute between Conan and NBC, and he now has a late night gig on TBS.